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Thursday, September 12, 2013

Headlines- MERS and MERS vaccine

Greetings,

In my previous blog-post, I put up about the publication of a potential vaccine candidate against influenza, which can be made in almost a weeks notice. Influenza is a problem that pops up every 3-4 years, with a new strain gaining world wide scientific attention. Probably this is the first time a novel coronavirus (nCoV), now called as MERS (Middle East respiratory syndrome coronavirus) gained almost equivalent popularity as that of Influenza. Clearly lot of research is focussed on this problem. Once again a vaccine candidate has grabbed my attention, and so this post.

Coronaviruses (CoV; member of nidovirales), are so named for their appearance. The virus is enveloped, genetic material containing a single stranded RNA (+ sense) and a helical symmetry nucleocapsid. Human coronavirus infections with majority of them presenting as asymptomatic respiratory infection, and rarely a few causing moderate to severe illness. The coronaviruses are sub-classified into alpha (Group I), beta (Group II), gamma (Group III) , and delta (Group IV) coronaviruses. The most famous of the coronavirus is SARS (Severe acute respiratory syndrome- CoV). Other well studied human pathogens include HCoV-229E and HCoV-OC43.

MERS coronavirus, previously referred as Novel CoV (nCoV), is a lineage C member of the Beta- coronavirus group. The first case was seen on 31st May 2012, laboratory confirmed in Italy. The Index case was a previously healthy man, who had returned to Florence following a 40-day holiday in Jordan. He had signs of pneumonia, with bilateral lung infiltrates. In the following days, two more cases appeared, from people in close contact with the index case. The laboratory confirmation was based on a Real-Time PCR for genome upstream of the E gene (upE region). This was later also confirmed by National Influenza Center. There has been regular reporting of MERS CoV infection since. A week ago, WHO recorded a total of 114 global cases, of which 54 expired. On the date of this blog being posted, 16 more have been reported by Saudi Arabia's health ministry, making it to a total of 130 cases, of which 57 have been fatal (Update from CIDRAP; Link).

A great deal of work is currently in process, as to identification of the source. A wide range of serological test for detecting sero-conversion in animals has been done. Antibodies have been detected against the spike of MERS and has lead researchers to conclude that MERS has at some point of time, passed into camels. A study by Lipkin's group showed that they could isolate a small sequence(190 nt sequence, with maximum possibility of identity) in a faecal sample from an Egyptian tomb bat. They identified only small sequence just once. However, the press nailed it as "confirmed transmission", which is not agreed upon by the author itself (Listen to an interview with Lipkin in TWiV 247; Link). A quote from author “Although this fragment means a very close relative of the human MERS-CoV is found in a bat geographically close to the first case, the fact it is identical in this short region doesn’t mean that these bats are the direct source of the human case.” Source. Other comments include “It can absolutely not be ruled out that it is a sequence derived not from MERS-CoV but from another, closely related MERS-CoV like virus”. and "The finding is, of course, important, but it has to be reproduced by others, and it has to be found in other bats.” Source

The basic biology of CoV is very much similar to SARS CoV. Most of the studies done and mechanisms that has been worked out, has been guided from findings of SARS virus. From the DNA sequences that have been studied, it has been estimated that there are 10 open reading frames, expressed through nested set of 8 mRNAs.

Of all, the most important finding was the cellular receptor- dipeptidyl peptidase 4 (DPP4). The DPP4 binds to a 231-residue region in the spike (S) protein of MERS-CoV. The RNA genome is pumped in through a plasma or endosomal membrane fusion, into the target cell. The RNA immediately transcribes to proteins and RNA, which is packaged and released. See Fig 2.

As you would gather from the above, the number of reported cases isn't huge of a deal. But what has been the issue is case fatality rate. The virus is now slowly opening up to a spreading potential. The best thing that a pathogen can have is ability to spread well, which can be well inferred from the current trend. Clearly we need a vaccine.

When I say vaccine, the first thing that I can think about is antibodies against Spike protein. A genetically stable modified strain Vaccinia Virus Ankara (MVA) expressing full-length spike protein (MVA-MERS-S) has been constructed that induces neutralizing antibodies in mice model. I don't know much of the details about this as I haven't read the paper. Sorry, but I don't have access. But I can vaguely assume that it is a sensible approach. Probably, we could also try sub- component vaccine Just the Spike protein expressing only the "231-residue binding region" would have any improved effect? (Am assuming that just the epitope would be more specific. I have no idea if that would have any alternate or better results).

Fig 3: Growth kinetics of the deletion mutants.

But that hasn't somehow made the headlines. But what did make a news is an engineered strain lacking the E gene (rMERS-CoV-E) that can replicate, but not propagate. The research story reads so. Mutations created in 3, 4a, 4b and 5 (accessory genes), had no significant effect on MERS- CoV replication and showed similar growth rates as that of wild- type virus. See Fig 3 taken from paper. But when the E gene was mutated there was a clear disability. The virus was replication competent but propagation defective. This is mostly due to requirement of E to mature.

A question that arises here for me is "Does deleting E defeat the whole purpose of live attenuated vaccine?". The bang point of Live vaccines are that they are self replicating and hence doesn't need to be given multiple antigen doses. But since loss of E essentially cripples MERS virus and dies of quickly after producing some antigens. (Hmmm!!! Probably, I can give some more thought into this). Oh wait. So how did the researchers grow this strain then? Ah, simple. They integrated E gene into host chromosome (Which is not borrowed by virus), and so these "packaging cells", will support growth. However the new grown virus still doesn't possess E gene.

Photo 2: Camel

Of course, we wont have a vaccine say in about next month. There is this triple hurdle rule (make at least 3 mutations in genes to avoid immediate recombinant revertants) to consider the use of genetically modified live attenuated vaccine for commercial clinical use. So it comes as no surprise to me when Dr. Peter Hotez comments, "There's a long way to go from showing in a research laboratory that you've got a potential candidate vaccine to actually producing a bottle of that vaccine that is going to be used or stockpiled". The same source says, an animal oriented vaccine (Maybe camels are the best candidate to start in the first place), maybe manufactured soon and deployed into the field. Source.

But if camels are supposed to be mediating the spread is it appropriate to vaccinate them without the triple gene defect protection? I mean it could revert in animal and pass onto humans. I leave the debate to the reader.